The invention relates to an electrooptical display element with a very narrow dependence of the contrast on the viewing angle and a particularly steep electrooptical characteristic line.
The properties of nematic or nematic-cholesteric liquid crystal materials are utilized for liquid crystal display elements for significant modification of their optical properties, such as light transparency, light scattering, birefringence, reflectance or colour, under the influence of electrical fields. Functioning of such display elements is based here, for example, on the phenomenon of dynamic scattering, deformation of aligned phases, the Schadt-Helfrich effect in the twisted cell or the SBE effect.
Amongst these customary types of liquid crystal display elements, display elements with a large surface area, thin construction and a low power consumption and capable of presenting large quantities of information have in particular recently gained increasing importance. Highly informative display elements of this type are required in electronic data processing, office automation--this includes, inter alia, electronic word processing--the video industry and, in the not too distant future, also the car industry. Because of their properties, liquid crystal displays are predestined for these display systems. Highly informative liquid crystal displays which have a large surface area and use nematic liquid crystals are operated by the so-called time multiplex method. This control results in displays with a good contrast which is independent of the viewing angle only if the electrooptical characteristic line of the display is very steep. Until liquid crystals with an extremely small ratio of the elastic constants for bending and spreading K.sub.3 /K.sub.1 are available, there are limits to the steepness of the electrooptical characteristic line of a TN cell. As is known, the contrast can be improved by increasing the twist angle, which in the TN cell is 90.degree. (European Published Application No. 0,098,070; C. M. Waters, E. P. Raynes and V. Brimmel, Mol. Cryst. Liq. Cryst. 123 (1985) 303; German Offenlegungsschrift DE No. 3,423,993 A1; T. J. Scheffer and J. Nehring, J. Appl. Phys. 58 (1985) 3022; T. J. Scheffer and J. Nehring, Appl. Phys. Lett. 45 (1984) 1021). Whilst the influence of the various liquid crystal parameters on the steepness of the characteristic line and thus on the contrast is known for the TN cell, there is still substantial uncertainty as to the importance of some liquid crystal properties in the case of display elements with a twist angle &gt;90.degree.. The influence of the elastic constants on the contrast of a display element with a higher twist angle is not discussed at all by T. J. Scheffer et al., and is discussed by C. M. Waters et al. only with very greatly simplified assumptions.
There are therefore still considerable difficulties in the practical application of supertwist cells (TN cell with a twist angle &gt;90.degree., in particular values from 150.degree. to 250.degree., preferably in accordance with the literature references described above). These include, above all, the realization of a steep electrooptical characteristic line and the associated good contrast, which moreover should as far as possible be independent of the viewing angle.